The two RCA connectors are labeled Input and Xover, but these labels are somewhat misleading. The Xover input actually bypasses the internal crossover and level controls, while Input does not. According to the manual, Xover should be used if you have an external crossover, while the other input should be used with an A/V receiver or processor. However, I would say that Xover should be used whenever the source (receiver, processor, or crossover) includes bass management with a crossover at or below 100Hz; that way, the sub's level and input-frequency range are controlled from the source. Besides, you should avoid having two filters in the signal path, if possible.